此页面尚未翻译为您的语言。 请求翻译

GetItem

PDF

The GetItem operation returns a set of attributes for the item with the given primary key. If there is no matching item, GetItem does not return any data and there will be no Item element in the response.

GetItem provides an eventually consistent read by default. If your application requires a strongly consistent read, set ConsistentRead to true. Although a strongly consistent read might take more time than an eventually consistent read, it always returns the last updated value.

Request Syntax

{
   "AttributesToGet": [ "string" ],
   "ConsistentRead": boolean,
   "ExpressionAttributeNames": { 
      "string" : "string" 
   },
   "Key": { 
      "string" : { 
         "B": blob,
         "BOOL": boolean,
         "BS": [ blob ],
         "L": [ 
            "AttributeValue"
         ],
         "M": { 
            "string" : "AttributeValue"
         },
         "N": "string",
         "NS": [ "string" ],
         "NULL": boolean,
         "S": "string",
         "SS": [ "string" ]
      }
   },
   "ProjectionExpression": "string",
   "ReturnConsumedCapacity": "string",
   "TableName": "string"
}

Request Parameters

The request accepts the following data in JSON format.

Note

In the following list, the required parameters are described first.

Key

A map of attribute names to AttributeValue objects, representing the primary key of the item to retrieve.

For the primary key, you must provide all of the attributes. For example, with a simple primary key, you only need to provide a value for the partition key. For a composite primary key, you must provide values for both the partition key and the sort key.

Type: String to AttributeValue object map

Key Length Constraints: Maximum length of 65535.

Required: Yes

TableName

The name of the table containing the requested item. You can also provide the Amazon Resource Name (ARN) of the table in this parameter.

Type: String

Length Constraints: Minimum length of 1. Maximum length of 1024.

Required: Yes

AttributesToGet

This is a legacy parameter. Use ProjectionExpression instead. For more information, see AttributesToGet in the Amazon DynamoDB Developer Guide.

Type: Array of strings

Array Members: Minimum number of 1 item.

Length Constraints: Maximum length of 65535.

Required: No

ConsistentRead

Determines the read consistency model: If set to true, then the operation uses strongly consistent reads; otherwise, the operation uses eventually consistent reads.

Type: Boolean

Required: No

ExpressionAttributeNames

One or more substitution tokens for attribute names in an expression. The following are some use cases for using ExpressionAttributeNames:

• To access an attribute whose name conflicts with a DynamoDB reserved word.

• To create a placeholder for repeating occurrences of an attribute name in an expression.

• To prevent special characters in an attribute name from being misinterpreted in an expression.

Use the # character in an expression to dereference an attribute name. For example, consider the following attribute name:

• Percentile

The name of this attribute conflicts with a reserved word, so it cannot be used directly in an expression. (For the complete list of reserved words, see Reserved Words in the Amazon DynamoDB Developer Guide). To work around this, you could specify the following for ExpressionAttributeNames:

• {"#P":"Percentile"}

You could then use this substitution in an expression, as in this example:

• #P = :val

Note

Tokens that begin with the : character are expression attribute values, which are placeholders for the actual value at runtime.

For more information on expression attribute names, see Specifying Item Attributes in the Amazon DynamoDB Developer Guide.

Type: String to string map

Value Length Constraints: Maximum length of 65535.

Required: No

ProjectionExpression

A string that identifies one or more attributes to retrieve from the table. These attributes can include scalars, sets, or elements of a JSON document. The attributes in the expression must be separated by commas.

If no attribute names are specified, then all attributes are returned. If any of the requested attributes are not found, they do not appear in the result.

For more information, see Specifying Item Attributes in the Amazon DynamoDB Developer Guide.

Type: String

Required: No

ReturnConsumedCapacity

Determines the level of detail about either provisioned or on-demand throughput consumption that is returned in the response:

• INDEXES - The response includes the aggregate ConsumedCapacity for the operation, together with ConsumedCapacity for each table and secondary index that was accessed.

  Note that some operations, such as GetItem and BatchGetItem, do not access any indexes at all. In these cases, specifying INDEXES will only return ConsumedCapacity information for table(s).

• TOTAL - The response includes only the aggregate ConsumedCapacity for the operation.

• NONE - No ConsumedCapacity details are included in the response.

Type: String

Valid Values: INDEXES | TOTAL | NONE

Required: No

Response Syntax

{
   "ConsumedCapacity": { 
      "CapacityUnits": number,
      "GlobalSecondaryIndexes": { 
         "string" : { 
            "CapacityUnits": number,
            "ReadCapacityUnits": number,
            "WriteCapacityUnits": number
         }
      },
      "LocalSecondaryIndexes": { 
         "string" : { 
            "CapacityUnits": number,
            "ReadCapacityUnits": number,
            "WriteCapacityUnits": number
         }
      },
      "ReadCapacityUnits": number,
      "Table": { 
         "CapacityUnits": number,
         "ReadCapacityUnits": number,
         "WriteCapacityUnits": number
      },
      "TableName": "string",
      "WriteCapacityUnits": number
   },
   "Item": { 
      "string" : { 
         "B": blob,
         "BOOL": boolean,
         "BS": [ blob ],
         "L": [ 
            "AttributeValue"
         ],
         "M": { 
            "string" : "AttributeValue"
         },
         "N": "string",
         "NS": [ "string" ],
         "NULL": boolean,
         "S": "string",
         "SS": [ "string" ]
      }
   }
}

Response Elements

If the action is successful, the service sends back an HTTP 200 response.

The following data is returned in JSON format by the service.

ConsumedCapacity

The capacity units consumed by the GetItem operation. The data returned includes the total provisioned throughput consumed, along with statistics for the table and any indexes involved in the operation. ConsumedCapacity is only returned if the ReturnConsumedCapacity parameter was specified. For more information, see Capacity unit consumption for read operations in the Amazon DynamoDB Developer Guide.

Type: ConsumedCapacity object

Item

A map of attribute names to AttributeValue objects, as specified by ProjectionExpression.

Type: String to AttributeValue object map

Key Length Constraints: Maximum length of 65535.

Errors

For information about the errors that are common to all actions, see Common Errors.

InternalServerError

An error occurred on the server side.

HTTP Status Code: 500

ProvisionedThroughputExceededException

Your request rate is too high. The AWS SDKs for DynamoDB automatically retry requests that receive this exception. Your request is eventually successful, unless your retry queue is too large to finish. Reduce the frequency of requests and use exponential backoff. For more information, go to Error Retries and Exponential Backoff in the Amazon DynamoDB Developer Guide.

HTTP Status Code: 400

RequestLimitExceeded

Throughput exceeds the current throughput quota for your account. Please contact Support to request a quota increase.

HTTP Status Code: 400

ResourceNotFoundException

The operation tried to access a nonexistent table or index. The resource might not be specified correctly, or its status might not be ACTIVE.

HTTP Status Code: 400

Examples

Retrieve Item Attributes

The following example retrieves three attributes from the Thread table. In the response, the ConsumedCapacityUnits value is 1, because ConsistentRead is set to true. If ConsistentRead had been set to false (or not specified) for the same request, an eventually consistent read would have been used and ConsumedCapacityUnits would have been 0.5.

Sample Request

POST / HTTP/1.1
Host: dynamodb.<region>.<domain>;
Accept-Encoding: identity
Content-Length: <PayloadSizeBytes>
User-Agent: <UserAgentString>
Content-Type: application/x-amz-json-1.0
Authorization: AWS4-HMAC-SHA256 Credential=<Credential>, SignedHeaders=<Headers>, Signature=<Signature>
X-Amz-Date: <Date>
X-Amz-Target: DynamoDB_20120810.GetItem

{
    "TableName": "Thread",
    "Key": {
        "ForumName": {
            "S": "Amazon DynamoDB"
        },
        "Subject": {
            "S": "How do I update multiple items?"
        }
    },
    "ProjectionExpression":"LastPostDateTime, Message, Tags",
    "ConsistentRead": true,
    "ReturnConsumedCapacity": "TOTAL"
}

Sample Response

HTTP/1.1 200 OK
x-amzn-RequestId: <RequestId>
x-amz-crc32: <Checksum>
Content-Type: application/x-amz-json-1.0
Content-Length: <PayloadSizeBytes>
Date: <Date>
 {
    "ConsumedCapacity": {
        "CapacityUnits": 1,
        "TableName": "Thread"
    },
    "Item": {
        "Tags": {
            "SS": ["Update","Multiple Items","HelpMe"]
        },
        "LastPostDateTime": {
            "S": "201303190436"
        },
        "Message": {
            "S": "I want to update multiple items in a single call. What's the best way to do that?"
        }
    }
}

See Also

For more information about using this API in one of the language-specific AWS SDKs, see the following:

• AWS Command Line Interface

• AWS SDK for .NET

• AWS SDK for C++

• AWS SDK for Go v2

• AWS SDK for Java V2

• AWS SDK for JavaScript V3

• AWS SDK for PHP V3

• AWS SDK for Python

• AWS SDK for Ruby V3